1. Field of the Invention
The invention relates to a range of new products; processes for preparing them from dry biological polymers (biopolymers) using ionizing radiation in the solid-non fluid state in the presence of an unsaturated gas under specified reaction conditions and the uses thereof.
2. The Prior Art
It is known in the art to subject certain types of polymeric materials to irradiation in order to achieve a number of different goals, although to our knowledge it is not known in the art to subject such biopolymers to high energy irradiation in the presence of a mediating gas, e.g., acetylene, in order to modify the biopolymer so as to enhance its properties in one or more respects. The following U.S. Pat. No. 3,215,634 (Walker); U.S. Pat. No. 4,024,073 (Shimizu); U.S. Pat. No. 4,716,224 (Sakurai); U.S. Pat. No. 4,746,514 (Warne); U.S. Pat. No. 4,987,222; (De Ambrosi); and U.S. Pat. No. 5,376,692 (Park); and published foreign application WO 96/03147; (Fidia, S.p.A) are of interest, but are not significantly relevant. For example, none of the art teaches irradiation of polymeric materials in the solid state, including Warne (""514) who does use an ethylenically unsaturated compound.application WO 96/03147; (Fidia, S.p.A) are somewhat, but not significantly relevant, which is not a gas.
Sakurai (""224) teaches the cross-linking of hyaluronic acid with polyfunctional epoxy compounds under certain conditions, none of which teach the use of ionizing radiation/unsaturated [alkenic or alkynic] gases.
Walker (""634) and Shimizu (""073) also disclose the use of various chemical cross-linking agents for preparing cross-linked polysaccharide products.
De Ambrosi (""222) discloses the controlled preparation of low molecular weight glucosaminoglycans by depolymerizing high molecular weight glucosaminoglycans using xcex3 radiation.
Warne (""514) teaches away from the invention. Warne discloses the preparation of cross-linked hydrogels by subjecting a polysaccharide, specifically, nothing of higher molecular weight than a pentasaccharide, to ionizing radiation in the presence of an ethylenically unsaturated compound (but not a gas) having at least one hydrophilic group.
Park (""692) discloses proteins, e.g., albumin, that are functionalized so that when the albumin is bonded to a blood compatible substrate, and after treatment with radiation, free radicals formed on both the protein and the substrate chemically bind to one another. This reference does not teach or suggest cross-linking the polymers by using ionizing radiation in the presence of an unsaturated gas which forms part of the cross-link.
Fidia (PCT application no. WO 96/03147) teaches the synthesis of chemical gels from polyelectrolyte polysaccharides, including HA and HA-benzyl ester by xcex3-irradiation, preceded by functionalizing to introduce olefinic bonds, into the structure thereof. The only functionalizing agent disclosed is glycidyl acrylate. Other, less relevant foreign patents such as EP 000 038 426; JP 360,143,991; JP 363,301,234; JP 401,118,529; DE 004,123,889; DE 004,124,338; and JP 406,073,102 are noted.
Non-patent literature relating to the subject matter of this invention, particularly, certain studies conducted on some of the starting materials used herein and the effect of ionizing radiations on uncharged polysaccharides (such as starch and cellulose) and on polyelectrolyte polysaccharides (such as hyaluronic acid and its cross-linked derivative hylan, alginates, heparin etc) is to induce degradation, with main chain scission leading to a decrease in molecular weight and viscosity are discussed in:
The effect of sterilizing doses of xcex3-irradiation on the molecular weight and emulsification properties of gum arabic; Blake, et al, Food Hydrocolloids 1988, Vol.2 No.5, p.407-415; The effects of radiation on carbohydrates (Phillips, G., Chapter 26 pages 1217-1297 in xe2x80x9cThe Carbohydratesxe2x80x9d, second edition. (Eds. Ward Pigman/Derek Horton), Academic Press Inc. New York, 1980); Free radical formation and degradation of cellulose by ionizing radiations. (Nakamura et al. Polymer Photochemistry, 1985, 6, 135-159); Photochemistry and radiation chemistry of cellulose (Phillips et al. Cellulose Chemistry and Its Applications 1985, 290-311); Radiation effects on the biological activity and molecular weight parameters of heparin. (Edwards et al. Carbohydrate Polymers, 1985, 5, 473-478); The radiation-induced degradation of hyaluronic acid. (Deeble et al. Radiat. Phys. Chem. 1991, Vol.37, No.1, 115-118); Susceptibility of Connective Tissue: Biomaterials to Radiation. (Phillips et al. Journal of Korea Biomaterial Research Institute, Vol. 1, No. 1, August 1991, p.92); The enhanced stability of the cross-linked hylan structure to hydroxyl radicals compared with the uncross-linked hyaluronan. (Al-Assaf et al. Radiat. Phys. Chem. 1995, Vol 46, 207-217); Identification of radicals from hyaluronan (hyaluronic acid) and cross-linked derivatives using electron paramagnetic resonance spectroscopy. (Al-Assaf et al. Carbohydrate Polymers, 1999, Vol.38, 17-22); The role of the proteinaceous component on the emulsifying properties of gum arabic. (Randall et al. Food Hydrocolloids, 1988, 2, No.2, 131-140); Structural and chemical properties of gum arabic. Their practical impact. (Phillips et al. Proceedings Gum Arabic Symposium, ZDS, Solingen, Germany, Jun. 6-8, 1988); The influence of structure and technology on gum arabic functionality. (Phillips, G., Supplement to Food Review February/March, 1988, pp.64-68); Fractionation and characterization of gum from Acacia senegal. (Randall et al. Food Hydrocolloids, 1989, Vol.3, No. 1, p.65-75); The molecular characterization of the polysaccharide gum from Acacia senegal. (Osman et al. Carbohydrate Research, 1993, 246, pp. 303); The Classification of Natural Gums. Part III. Acacia senegal and Related Species (Gum Arabic); (Jurasek et al. Food Hydrocolloids, 1993, Vol.7, No.3, pp. 255-280); Acacia gum (Gum Arabic): a nutritional fibre; metabolism and calorific value. (Phillips, G., Food Additives and Contaminants, 1998, Vol. 15 No.3, 251-264); and. A review of recent developments on the regulatory, structural and functional aspects of gum arabic. (Islam et al. Food Hydrocolloids, 1997, Vol 11 (4), pp 357-365). Fractionation and characterization of gum from Acacia Senegal. (Randall et al. Food Hydrocolloids, 1989, Vol.3, pp.65-75) The molecular characterization of the polysaccharide gum from Acacia senegal. (Osman et al. Carbohydrate Research, 1993, 246, pp. 303); The Classification of Natural Gums. Part Ill. Acacia Senegal and Related Species (Gum Arabic) (Jurasek et al. Food Hydrocolloids, 1993, Vol.7, No.3, pp. 255-280).
The invention provides an extremely broad category of new biopolymers having dramatically improved properties in comparison with the starting biopolymers. The molecular weights of these materials can be increased in a controlled manner to provide new physical and chemical functionalities (for example emulsification and water binding). Aqueous solutions of the new products can be produced with literally any desired viscosity and/or viscoelasticity. The biopolymers can be converted into new hydrophilic gels (hydrogels) of defined particular size and having specified micromechanical properties. The changes can be accomplished without the introduction of new chemical substituents, and hence, the new materials retain the inherent biocompatibility of the starting, or parent biopolymer. One or more different biopolymers can be used together in the process to yield new bio-copolymers.
As used herein the term biopolymer and biological polymer is understood to mean a polymer derived from a biological source, whether plant, including microorganisms or animal.
The biopolymers contemplated by the invention comprise unsubstituted biopolymers extending over the entire field of plant and animal derived polysaccharides, whether charged or uncharged, as well as proteins directly derived from animal connective tissue sources such as collagen, gelatin, and from human and animal products, such as casein, combinations of one or more such polysaccharides with one or more proteins of plant originxe2x80x94such as arabinogalactan proteins, biological tissues and materials derived therefrom used for tissue replacement and transplantation, either finished or partially finished and which are made or formed from one or more of such biopolymers or combinations thereof with the other aforesaid materials. The biopolymers to be treated to form the new materials according to the process of the invention do not need to be modified in any way prior to treatment, for example, by introducing any functionalizing groups which might, in other processes be necessary to activate the biopolymer or make it more reactive.
Illustrative examples of the biopolymers contemplated by this invention include: acacia plant exudates, such as acacia senegal and acacia seyal, representing the arabinogalactan proteins present in all plants; dextran and related bacterial polysaccharides; chemically modified polysaccharides such as carboxymethyl cellulose; gelling polysaccharides from either bacterial origin (xanthan) or plant (carrageenan) or fruit origin (pectin); animal connective tissue polysaccharides and proteins, and combinations thereof, such as hyaluronan, proteoglycans and chemically modified animal derived polysaccharides such as hylan; and interactive combinations of these materials which can be associated, bonded and adhered in specific combinations.
In carrying out the process of the invention for producing the new materials from the starting biopolymers, it is preferred that the biopolymer be in its original solid state, i.e., dry, in an atmosphere comprising a mediating agent, preferably a low molecular weight unsaturated alkenic or alkynic gas such as ethylene, propylene or acetylene, preferably acetylene. Before introducing the mediating gas to the reaction site, the site must be flushed to remove therefrom any active, oxygen containing atmosphere. All the mediating gas is removed after completion of the process and therefore, the resulting new materials do not contain any of the mediating gas.
The biopolymeric system (or the finished or partially finished product made therefrom) from which the active atmosphere has been removed is then saturated with the mediating gas at atmospheric pressure and exposed to a source of ionizing radiation which may be either a radioactive isotope such as 60Co (xcex3-rays) or radiation generated by a high energy (250 KeV to 10 MeV) electron accelerator or X-rays generated by the accelerator or any other suitable device.
The minimum absorbed radiation dose may vary from 1 kGy to 50 kGy, depending on the structure of the biopolymer, whether branched or long-chain nature of the product desired, whether of increased molecular weight to form a readily water soluble product or to form either a gel or a membrane product. As a general guide, highly branched polysaccharide structures can produce a 4-fold increase in molecular weight with doses up to 10 kGy and gels with doses up to 50 kGy, whereas straight chain structures can yield a similar change with doses as low as 1-3 kGy. Proteins require doses up to 25 kGy to achieve a similar result. Blends and combined adhesive systems require careful dose selection according to the composition of the systems
Following the irradiation step in the presence of the gaseous mediating agent, and in order to remove any activated species produced by the radiation system, the resulting biopolymer system or new material is subjected to heat treatment (annealing) in the absence of oxygen at elevated temperatures ranging from 40xc2x0 C. to 120xc2x0 C. depending on the heat stability of the biopolymer system which is being modified. This annealing step may ideally be carried out in the presence of the unsaturated gaseous atmosphere or, alternatively, in the presence of an inert gas such as nitrogen or helium, or in a vacuum oven. The former can increase the amount of new product formation, and the latter provides a suitable mechanism for termination of the process.
Following the annealing step, any residual gaseous mediating agent is removed from the modified biopolymeric system by aerating the system, and if necessary, the application of a vacuum process to the treated polymer. This will depend on the retention ability of the material for the gas which depends on the porosity of the solid system.
The resulting new biopolymers obtained by the above-described treatment are characterized by changes in the following parameters when comparing the starting material with the new biopolymers:
a) molecular weight can be increased 4-5 fold;
b) increased water binding usually parallels that of the increase in molecular weight;
c) emulsion with droplet sizes of the order 1 xcexcm can be achieved using reduced concentrations of the biopolymers;
d) a range of changes in viscosity and viscoelasticity of up to 1000 fold; and
e) there is observed formation of hydrogels having particle sizes from 150-2000 xcexcm.
The molecular weight of the starting biopolymer can be increased in a controlled manner to provide a new generation of products with enhanced properties without losing the basic functionalities of the parent biopolymer. The increased molecular parameters allow greater water binding, improved physical functionalities, such as lower emulsion droplet formation, new binding capabilities and functions to other polymers, whether charged or uncharged, and better fabrication qualities for drug and small ion release.
Water soluble products of increased or decreased viscosity and/or viscoelasticity can be produced. Thus, new food, industrial and medical products can be produced.
Of singular significance to this invention is the fact that no significant or identifiable chemical changes are introduced into the structure of the biopolymer as a result of the process. Thus, any new product made according to the invention will be used in practice in substantially the same way as the parent biopolymer.
Hydrophilic gels (hydrogels) can be produced of a defined particle size and specified micro-mechanical properties.
The products are as biocompatible as are the parent biopolymers.
A wide range of new products can be obtained by changes in the process parameters and these are an integral part of the spirit and purpose of the present invention, and are included within the scope of the claims made herein. The process does not yield a single new product, or even a series of new products within a particular type of biopolymer, but rather, it offers the opportunity of producing a family of new products, each of which may be tailor made for specific applications.